In various industries, solutions are utilized or generated which contain metallic materials and other contaminates. For instance, in the metal plating industries, plating wastes are generated which typically contain heavy metals and cyanide. With the heightened environmental concerns that are prevalent today, disposal of these waste solutions has been a major problem.
With the amount of land available for industrial waste disposal decreasing, recent efforts have been directed toward treating the waste liquid to reduce the heavy metal and cyanide content thereof in order to produce a purified liquid which can be disposed of by ordinary means.
A common method currently used for removing heavy metals from solutions prior to their disposal is by chemical precipitation. This method has problems in that different metals precipitate under different conditions, particularly, at different pH levels. If more than one type of heavy metal is contained in the same waste stream, the optimum conditions for precipitation will often be different for the different metal ions. Additionally, chemical precipitation is of little, if any, benefit in reducing the non-metal contaminate content, such as cyanide content, of the waste stream.
Recently, a preferred method for reducing contaminates in a waste stream has been to introduce the waste stream into an electrolytic cell wherein the metal contaminates are deposited on one or more cathodes contained in the cell and the cyanide ions are oxidized at one or more anodes. However, electrolytic treatment of waste streams generates another set of problems. Specifically, in order to reduce the contaminate content of the waste stream to an acceptable level, it is necessary to perform electrolysis on the waste stream for a long period of time. Since the cost of energy is very high, the use of electrolysis to treat waste streams is not only time consuming, but also it can be relatively expensive. Additionally, the recovered metal has to be removed from the cathodes, and complicated cell designs result in extended periods during which the electrolytic cell cannot be used due to the removal, cleaning and replacement of the cathodes.
It is, therefore, a principal object of the present invention to provide a method and an apparatus for the batch treatment of a waste stream which reduces or eliminates the problems associated with prior art treatment methods.
It is a further object of the present invention to provide a method and an apparatus which economically reduces the contaminate content of a waste stream in a comparatively short period of time by utilizing an electrolytic cell of a specific configuration.
In the improved process and apparatus of this invention, there is provided a vessel or electrolytic cell having an inlet section, an electrode section having a plurality of anodes and cathodes in the form of graphite sheets, and an outlet section. Air agitation of the waste stream is provided at the inlet and outlet sections to cause turbulence in the waste stream being treated and to aid in the oxidation of the cyanide. The waste stream is continuously recirculated through the vessel by being removed from the outlet section, passed through a filter to remove any undissolved solids therefrom and then reintroduced into the inlet section. The combination of high electrode surface area, vigorous air agitation and recirculation of the waste stream enables the contaminate level of the waste stream unexpectedly to be reduced to an extremely low level in a short period of time. Additionally, the use of graphite sheets as the electrodes reduces the maintenance typically associated with electrolytic cells because the graphite sheets are extremely durable and they can be replaced with a minimum amount of down time.
Other objects and purposes of the invention will be apparent to persons familiar with apparatuses and processes of this general type, after reading the following specification and inspecting the accompanying drawings.